Credit is allowed on property included in the gross estate that was previously taxed within five years. The five-year period is computed from the date of death of the prior decedent and not from the date of payment of such tax. The amount of credit for previously paid tax is the lesser of the following:
the tax paid in the previous estate multiplied by the result of the total property amount transferred to the present estate divided by the taxable estate of the previous estate, or
the tax due in the present estate multiplied by the result of the total property amount received from the previous estate divided by the taxable estate of the present estate.